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We investigated changes in wildfire risk over the 1901À2002 (AD) period with an analysis of broad-scale patterns of July monthly drought code (MDC) variability on 28 forested ecoregions of the North American and Eurasian continents. The MDC is an estimate of the net effect of changes in evapotranspiration and precipitation on cumulative moisture depletion in soils, and is well correlated with annual fire statistics across the circumboreal (explaining 25-61% of the variance in regional area burned). We used linear trend and regime shift analyses to investigate (multi-) decadal changes in MDC and percentage area affected by drought, and kernel function for analysis of temporal changes in the occurrence rates of extreme drought years. Our analyses did not reveal widespread patterns of linear increases in dryness through time as a response to rising Northern Hemisphere land temperatures. Instead, we found heterogeneous patterns of drought severity changes that were inherent to the nonuniformly distributed impacts of climate change on dryness. Notably, significant trends toward increasing summer moisture in southeastern and southwestern boreal Canada were detected. The diminishing wildfire risk in these regions is coherent with widely reported decreases in area burned since about 1850, as reconstructed by dendrochronological dating of forest stands. Conversely, we found evidence for increasing percentage area affected by extreme droughts in Eurasia ( 1 0.57% per decade; Po0.05) and occurrence rates of extreme drought years in Eurasian taiga (centered principally on the Okhotsk-Manchurian taiga, P 5 0.07). Although not statistically significant, temporal changes in occurrence rates are sufficiently important spatially to be paid further attention. The absence of a linear trend in MDC severity, in conjunction with the presence of an increase in the occurrence rate of extreme drought years, suggest that fire disturbance regimes in the Eurasian taiga could be shifting toward being increasingly pulse dependent.

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The role of future forests in global biogeochemical cycles will depend on how different tree species respond to climate. Interpreting the response of forest growth to climate change requires an understanding of the temporal and spatial patterns of seasonal climatic influences on the growth of common tree species. We constructed a new network of 310 tree‐ring width chronologies from three common tree species (Quercus robur, Pinus sylvestris and Fagus sylvatica) collected for different ecological, management and climate purposes in the south Baltic Sea region at the border of three bioclimatic zones (temperate continental, oceanic, southern boreal). The major climate factors (temperature, precipitation, drought) affecting tree growth at monthly and seasonal scales were identified. Our analysis documents that 20th century Scots pine and deciduous species growth is generally controlled by different climate parameters, and that summer moisture availability is increasingly important for the growth of deciduous species examined. We report changes in the influence of winter climate variables over the last decades, where a decreasing influence of late winter temperature on deciduous tree growth and an increasing influence of winter temperature on Scots pine growth was found. By comparing climate–growth responses for the 1943–1972 and 1973–2002 periods and characterizing site‐level growth response stability, a descriptive application of spatial segregation analysis distinguished sites with stable responses to dominant climate parameters (northeast of the study region), and sites that collectively showed unstable responses to winter climate (southeast of the study region). The findings presented here highlight the temporally unstable and nonuniform responses of tree growth to climate variability, and that there are geographical coherent regions where these changes are similar. Considering continued climate change in the future, our results provide important regional perspectives on recent broad‐scale climate–growth relationships for trees across the temperate to boreal forest transition around the south Baltic Sea.

Summary 1.Forest fires are one of the main disturbance agents in boreal and temperate ecosystems. To decipher large-scale temporal and spatial patterns of past fire activity in Scandinavia, we analysed the synchronicity of dendrochoronologically reconstructed fire events in a large network of sites (n = 62; 3296 samples, 392 individual fire years) covering a wide geographical gradient (56.5-67. 0°N and 9.3-20.5°E) over AD 1400-1900. We identified large fire years (LFY) as years with regionally increased forest fire activity and located the geographical centres of climatic anomalies associated with synchronous LFY occurrence across the region, termed LFY centroids. 2. The spatial pattern of LFY centroids indicated the presence of two regions with climatically mediated synchronicity of fire occurrence, located south and north from 60°N. The return intervals of LFYs in Scandinavia followed a Weibull distribution in both regions. Intervals, however, differed: a period of 40 years would carry a 0.93 probability of LFY occurrence in the southern region, but only a 0.48 probability of LFY occurrence in the northern region. 3. Over 1420-1759, the northern region was characterized by significantly higher temporal variability in LFY occurrence than the southern region. Temporal correlation of LFYs with reconstructed average summer temperature and total precipitation was evident mainly for the northern region. LFYs in this region were associated with positive temperature and negative precipitation anomalies over Scandinavia and with colder and wetter conditions in more southern parts of the European subcontinent. 4. Synthesis. Historical patterns of the occurrence of large fire years (LFY) in Scandinavia point towards the presence of two well-defined zones with characteristic fire activity, with the geographical division at approximately 60°N. The northern and mid-boreal forests, although exhibiting lower LFY frequencies, appeared to be more sensitive to past summer climate, as compared to the southern boreal forests. This would imply that fire regimes across Scandinavia may show an asynchronous response to future climate changes.

SignificanceThe metabolism of North America’s oldest boreal trees (Thuja occidentalis L.) is strongly affected by rising anthropogenic normalCnormalO2 emissions. Intrinsic water use efficiency (italiciWUE) increased dramatically, although nonlinearly, since the beginning of the industrial Era. Our study shows that while T. occidentalis L. acclimated to rising [normalCnormalO2], no stem growth was observed, suggesting that trees could not benefit from the increased italiciWUE.

Reconstruction of a 253-year long mast record of European beech reveals its association with large scale temperature variability and no long-term trend in mast frequencies. Agricultural and forest meteorology. 31MYs were strongly associated with both the 500 mb height anomalies and average summer temperatures 32 during two years preceding a MY: a mast year (t) followed a cold summer two years (t-2) prior to the mast 33 year and a warm summer one year prior (t-1) to the mast year. During t-2 years, the geographical pattern of 34 500 mb height anomalies exhibited a strong height depression in the region centered in the Northern Sea 35 and extending towards eastern North America and statistically significant (p < 0.05) temperature anomalies 36 covering predominantly southern Scandinavia (area below 60 N) and British Isles. A year immediately 37 preceding a mast year (t-1) was characterized by a strong regional high pressure anomaly centered in 38 southern Scandinavia with significant temperature anomalies extended mostly over southern Scandinavia 39 and Germany. 40The long-term mean MY return interval was 6.3 years, with 50 and 90% probabilities of MY occurrence 41 corresponding to 6 and 15 years, respectively. Periods with intervals significantly shorter than the long-42 term mean were observed around 1820 -1860 and 1990 -2006 (means -3.9 and 3.2 years, respectively). 43However, the difference in return intervals between two sub-periods themselves was not significant. 59Fagaceae family (Hiroki and Matsubara, 1995; Hilton and Packham, 2003). At tree level, such events 60 imply large shifts in resource allocation towards reproductive organs, suggesting trade-offs between seed 61 production and biomass accumulation (Monks and Kelly, 2006; Drobyshev et al., 2010). At the stand and 62 regional levels, mast years are important for species regeneration and subsequent canopy dynamics 63 (Emborg, 1998; Frey et al., 2007; Barna, 2011), as well as for dynamics of animal species utilizing beech 64 seeds as a food resource (Schnurr et al., 2002; Clotfelter et al., 2007; Jensen et al., 2012). Mast seeding, 65 specifically of Fagus spp., has been widely acknowledged in forestry as a way to promote natural tree 66 regeneration on clearcut areas (Henriksen, 1988;Övergaard et al., 2007; Bileik et al., 2009). 67Mast years in European beech (Fagus sylvatica L.) have been shown to be strongly affected by annual 68 climatic variability. Temperature dynamics apparently plays the major role in controlling mast events 69 (Piovesan and Adams, 2001). Warm and dry conditions were typically observed during the summers 70 preceding the mast year, and cold summers with sufficient amount of precipitation were often observed two 71 years prior to a mast year. A study in southern Sweden has revealed a strong effect of temperature on beech 72 masting behavior (Drobyshev et al., 2010). In line with these findings, physiological studies have 73 repeatedly pointed to European beech as a temperature sensitive species, e.g. relative to the onset of t...

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